Adachiite is a rare member of the tourmaline group, chemically characterized by its calcium- and iron-rich composition. It typically presents as dark, prismatic crystals found in association with metamorphic assemblages in Japan, requiring professional analysis to distinguish from more common iron-rich tourmalines like schorl.

Hardness

Mohs

Transparency

Transparent

mineralrareTourmaline Group family

Adachiite

Hardness: 7
Transparency: Transparent

No image

Is this adachiite?

3-step field check

Run through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.

1
Test the hardness
Try to scratch adachiite with a known reference. Adachiite sits at Mohs 7, softer than the next harder reference, harder than the previous one.
2
Match the color range
Compare against the expected color range: brownish purple, bluish purple, black massive aggregates.
3
Look at form & habit
Crystal system: trigonal. Typical habit: prismatic crystals with rounded triangular cross-sections.

Often confused with

Adachiite vs. its common look-alikes, and how to tell them apart in the field.

Often found alongside adachiite

Minerals reported to co-occur with adachiite. Spotting these in float or country rock is a strong cue you are in the right ground.

All properties

Chemical formula: CaFe₃Al₆(Si₅AlO₁₈)(BO₃)₃(OH)₃(OH)
Mohs hardness: 7
Density: 3.228 g/cm³
Colors: Brownish purpleBluish purpleBlack massive aggregates
Transparency: Transparent
Crystal system: Trigonal
Crystal habit: Prismatic crystals with rounded triangular cross-sections
Rarity: Rare
Uses: Collector
Host rock: In a hydrothermal vein that cuts a lateritic metamorphic emery (corundum and hercynite) deposit

Where rockhounds find adachiite

Classic worldwide localities

Nabagasako adit, Kiura mine, Saiki City, Oita Prefecture, Japan

Field-hunting tip

Where it forms: In a hydrothermal vein that cuts a lateritic metamorphic emery (corundum and hercynite) deposit. If you start seeing margarite, chlorite, diaspore in float, you are in the right ground. Typical habit: prismatic crystals with rounded triangular cross-sections. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify adachiite?+

Mohs hardness is 7. Common colors include brownish purple, bluish purple, black massive aggregates.

Where is adachiite found?+

Notable localities include Nabagasako adit, Kiura mine, Saiki City, Oita Prefecture, Japan.

What rocks look like adachiite?+

Adachiite is most often confused with Schorl, Feruvite. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with adachiite?+

Adachiite commonly co-occurs with Margarite, Chlorite, Diaspore, Schorl. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does adachiite form in?+

Where it forms: In a hydrothermal vein that cuts a lateritic metamorphic emery (corundum and hercynite) deposit. Working float back to the host body is the standard way to chase a fresh occurrence.

What is adachiite used for?+

Adachiite is used in collector.

Find adachiite on the map

RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.